a chamber having an internal space configured for performance of a wet process therein;

a chuck in a lower region of the internal space;

a probe having one end above an upper portion of the chuck, and including an electro-optical crystal having an optical refractive index configured to change in proportion to a magnitude of an electric field applied at an upper portion of a semiconductor substrate loaded onto the chuck, and the probe further including a reflective mirror on one surface of the electro-optical crystal;

a measuring unit connected to the probe, the measuring unit being configured to provide reference light to the probe and to detect a polarization component of reflected light from the reference light that is reflected from the reflective mirror of the probe; and

a controller configured to calculate an amount of electrostatic charge on a surface of the semiconductor substrate from the polarization component detected by the measuring unit.